I. Field of the Invention
This invention relates generally to a device referred to as a load cell used for measuring a force by means of a strain gage which device is typically used in a scale, and more specifically, to a load cell having a means for adjusting for weight shift or off center load adjustment.
II. Description of the Prior Art
There are many environments in which load cells are used for the purpose of determining forces applied to an object. One such environment is a weighing scale, such as a postal scale, wherein the strain of a load cell is measured to determine the weight of an object that is placed on a load support. An example of such a weighing scale that uses load cells may be found in U.S. Pat. No. 4,037,675.
It is known in the art to use in a scale a load cell of the parallelogram type. This type of structure was the outgrowth of a need for improved measuring techniques previously offered by mechanical scales that functioned by the use of beams on knife edges. These mechanical scales also utilized balancing forces with known weights or deflecting coil springs. The scales utilizing weight balancing techniques were accurate, but very cumbersome to use, while the spring scales did not have the desired accuracy for near precision type measurements.
Furthermore, because most of today's enterprises require the use of computers, it became desirable to have the weight of an object indicated in electrical or digital form. This digital readout was then utilized for data processing such as in the calculation of postage due on items mailed for various mailing zones. Since known mechanical scales could not provide these needs, the development of load cell technology has aided to fill this void.
Many known designs for load cells are in the form of a parallelogram type load transmitting structure. See, for example, U.S. Pat. Nos. 4,181,011 and 4,196,784. However, various problems have been encountered when using this type of structure in a scale. For example, some of these structures are relatively expensive to manufacture. In some, internal forces caused by the method of production or by fabrication and temperature gradients have affected their accuracy. The optimum performance of the load cell in many instances was also thwarted by inherent forces that were difficult to compensate for. In addition, the accuracy is effected to a large degree by the position that the weight to be measured is placed on the weighing pan of the scale. Although load cells have been successfully utilized in many commercial scale applications, there remains a real need in scales which employ a load cell for a device, that in addition to resolving many of the above-mentioned disadvantages, can easily correct or compensate the load cell for extraneous forces from weight placement deviations from a central loaded condition. Examples of other typical load cells are disclosed in U.S. Pat. Nos. 4,037,675, 4,103,545 and 4,170,270.